In recent years, nitrogen oxides in exhaust gas emitted from the internal combustion engines of power generators and automobiles into the atmosphere has been seen as a problem from the viewpoint of harmful emissions control. Nitrogen oxides become a cause of acid rain and photochemical smog, and there is movement to regulate emissions thereof globally. Due to internal combustion engines such as diesel engines and gasoline lean burn engines performing lean combustion, oxygen exists in abundance in the exhaust thereof. Among the harmful components existing in the exhaust of internal combustion engines, purification of nitrogen oxides progresses via a reduction reaction; however, since causing a reduction reaction to progress in exhaust having a high oxygen partial pressure is difficult, a variety of methods for solving this problem have been investigated.
For example, as a method to purify nitrogen oxides contained in exhaust from a lean burn internal combustion engine, a technique has been known in which nitrogen oxides are temporarily absorbed and adsorbed by passing through a nitrogen oxide storage and adsorption catalyst under lean conditions in which the exhaust becomes oxygen excessive, followed by forming a rich state in which the oxygen concentration in the exhaust gas is low by temporarily increasing the fuel injection amount, whereby the nitrogen oxides thus absorbed and adsorbed are reduced, and a variety of methods using this have been proposed.
A first method of the technique employs a nitrogen oxide absorption and purification catalyst made by combining an alkali metal such as potassium and barium, alkali earth metal, etc. with platinum, etc. This is a method in which nitrogen oxides are oxidized and absorbed on a nitrogen oxide absorption and purification catalyst in the form of NO3− using oxygen under a lean condition, and oxygen is becomes scarce in the exhaust gas by subsequently controlling the internal combustion engine to a rich condition, and a state in which carbon monoxide and hydrocarbons are abundant is temporarily formed, while nitrogen oxides absorbed while lean are catalytically reduced and purified with high efficiency using the carbon monoxide and hydrocarbons thus emitted under the reducing atmosphere while rich (for example, refer to Non-patent Document 1, Patent Document 1, and Patent Document 2).
A second method of the technique employs an adsorption and purification catalyst made by combining ceria, platinum, a solid acid, etc. This is a method in which nitrogen oxides in the form of NO2 are oxidized and adsorbed on the adsorption and purification catalyst using oxygen under a lean condition (refer to reaction formulas 1 to 3), followed by oxygen in the exhaust gas becoming scarce by controlling the internal combustion engine to a rich condition, and hydrogen is produced by causing carbon monoxide to react with water contained in the exhaust (refer to reaction formula 4). Furthermore, the hydrogen thus produced and nitrogen oxides are caused to react, when the nitrogen oxides adsorbed are removed, ammonia is produced simultaneously, and the ammonia thus produced is stored on the adsorption and purification catalyst (refer to reaction formula 5). When exposed again to a lean atmosphere, the nitrogen oxides in the exhaust are efficiently reduced and purified by way of causing the ammonia being stored and nitrogen oxides in the exhaust gas to react (refer to reaction formulas 6 to 8) (for example, refer to Non-patent Document 2 and Patent Document 3).NO→NO(ad)  formula 12NO+O2→2NO2(ad)  formula 2NO2→NO2(ad)  formula 3CO+H2O→H2+CO2  formula 4 (Water gas shift reaction)5H2+2NO→2NH3(ad)+2H2O  formula 54NH3+4NO+O2→4N2+6H2O  formula 6 (Low reactivity)2NH3+NO2+NO→2N2+3H2O  formula 7 (High reactivity)8NH3+6NO2→7N2+12H2O  formula 8[In the formula, (ad) represents adsorption.]
Furthermore, as a method separate from the above, a method has been investigated, based on the first method, in which a hydrogen enrichment means, which is provided upstream of a nitrogen oxide absorption and purification catalyst composed of platinum or the like, containing an alkali metal and alkali earth metal and disposed inside an exhaust channel, and hydrogen containing gas synthesized thereby is introduced to the nitrogen oxides absorption and purification catalyst disposed downstream (for example, refer to Patent Document 4). This is a method having a different approach than the first and second methods described above because, instead of the carbon monoxide and hydrocarbons being used as a reducing agent during purification of nitrogen oxides while rich as in the first method, hydrogen is produced in an upstream hydrogen enrichment means using the carbon monoxide and hydrocarbons, and the hydrogen is used as a reducing agent for nitrogen oxides.
Furthermore, as a method separate from the above, there is a technique that enables selective reduction of nitrogen oxides in a lean atmosphere from a low temperature using hydrogen. This is a technique that uses each of palladium in a metal active species, vanadia, titania, and alumina in an oxide carrier (for example, refer to Non-patent Document 3), as well as platinum in a metal active species, and zirconia in an oxide carrier (for example, refer to Non-Patent Document 4), and makes it possible to effectively purify nitrogen oxides from a low temperature with hydrogen as a reducing agent under an oxygen excessive atmosphere in both.    Patent Document 1: Japanese Patent No. 2586738    Patent Document 2: Japanese Patent No. 2600492    Patent Document 3: WO2005/044426    Patent Document 4: Japanese Patent No. 3642273    Patent Document 5: Japanese Unexamined Patent Application Publication No. H6-219721    Patent Document 6: Japanese Unexamined Patent Application Publication No. H7-313877    Non-patent Document 1: “Development of NOx Storage Reduction Three-way Catalyst System,” Collective Papers of Society of Automotive Engineers of Japan, Vol. 26, No. 4, October 1995    Non-patent Document 2: “A NOx Reduction System Using Ammonia Storage-Selective Catalytic Reduction in Rich and Lean Operation,” 15. Aachener Kolloquium Fahrzeug-und Motorentechnik 2006, pp. 259-270    Non-patent Document 3: Qi, G. et al.; “Selective Catalytic Reaction of Nitric Oxide with Hydrogen over Pd-based Catalysts,” Journal of Catalysis, 237, pp. 381-392 (2006)    Non-patent Document 4: Nanba, T. et al.; “Improvements in the N2 Selectivity of Pt catalysts in the NO—H2—O2 Reaction at Low Temperature,” Applied Catalysis B: Environmental, 46, pp. 353-364 (2003)